1. Field of the Invention
The present invention relates to an image pickup apparatus with a plurality of pixels for converting converged light into electric signals.
2. Related Background Art
A conventional solid-state image pickup device has photoelectric conversion elements such as photodiodes for converting received light into electric signals and microlenses for converging light on photoelectric conversion elements, as described in, for example, Japanese Patent Application Laid-open No. 05-040201.
A microlens is provided for preventing the sensitivity of a photodiode from being lowered by the amount of light reduced because of a smaller size of a recent ultra fine pixel.
FIG. 1A is a plan view of a conventional solid-state image pickup device. FIG. 1B is a cross sectional view of each pixel of the solid-state image pickup device shown in FIG. 1A. In FIGS. 1A and 1B, reference numeral 1 represents a pixel having a photodiode 5 formed in the surface layer of a silicon substrate (Si substrate) 18. Reference numeral 2 represents a light shielding layer for shielding the area of the pixel 2 excepting the photodiode 5. Reference numeral 3 represents an opening area through which light is incident upon the photodiode 5. Reference numeral 4 represents a microlens for converging light on the photodiode 5. Reference numeral 6 represents a color filter layer of red, green, blue or the like.
As shown in FIG. 1A, the conventional solid-state image pickup device has a plurality of pixels disposed in a predetermined pattern. As shown in FIG. 1B, opening areas 3 and microlenses 4 are formed at the same pitch as that of light reception areas of the photodiodes 5 of pixels 1. The optical axis of light converged by the microlens 4 is coincident with the center of gravity of the opening area 3. Light passed through the microlens 4 is therefore converged on generally the center of the light reception area of the photodiode 5.
Since the position of the microlens 4 is set to the position corresponding to that of the photodiode 5 of each pixel 1, it is possible to prevent the sensitivity of the photodiode 5 from being lowered by the opening area 3 reduced by a smaller size of a ultra fine pixel 1, because light is converged by the microlens 4.
With the conventional techniques, however, a portion of light converged by the microlens may not reach the photodiode, depending upon the position of a pixel of the solid-state image pickup device. A variation of light reception sensitivities of solid-state image pickup devices may occur in some cases.
FIGS. 2A and 2B are diagrams illustrating the reason of such a variation. In FIGS. 2A and 2B, reference numeral 10 represents an object to be taken with a solid-state image pickup device, and reference numeral 11 represents an image pickup lens for focussing light from the object 10 upon the solid-state image pickup device. In FIGS. 2A and 2B, like elements to those shown in FIGS. 1A and 1B are represented by using identical reference numerals.
In FIGS. 2A and 2B, (ii) represents a pixel disposed near to the center of the solid-state image pickup device, and (i) and (iii) represent pixels disposed in the peripheral area of the solid-state image pickup device.
As shown in FIG. 2A, light from the object 10 passes through the image pickup lens 11 and is focussed upon the solid-state image pickup device. Light from the object 10 applied to the pixel (ii) in FIG. 2B passes through the microlens 4 and becomes incident upon the photodiode 5.
Alter light from the object 10 applied to the pixels (i) and (iii) shown in FIG. 2B passes through the microlens 4, a portion of the light is intercepted by the light shielding area of the light shielding layer 2 and does not enter the photodiode 5. It is therefore impossible to eliminate a variation in light reception sensitivities of photodiodes near to and away from the optical axis of the image pickup lens.
FIG. 3 is a graph showing output signals from the solid-state image pickup device shown in FIGS. 2A and 2B. As shown in FIG. 3, a difference between the maximum and minimum values of output signals is 10% or larger of an average value of the output signals of the conventional solid-state image pickup device.
Assuming that the average of output signals of a conventional solid-state image pickup device is 100 mV, the maximum output signal is 105 mV or higher and the minimum output signal is 95 mV or lower. Generally, if a difference between the maximum and minimum values of output signals of a solid-state image pickup device is 10% or smaller than that of the average value, it is considered that this difference will not adversely affect the quality of a reproduced image.